ABSTRACT
Hydrothermal processes are considered as green, renewable, and sustainable for biofuel and bioenergy production. Residual moisture in biomass plays dual role both as solvent and as catalyst during hydrothermal processes. Therefore, expensive drying of wet biomass feedstock can be avoided. Thermodynamic properties of sub-and supercritical water vary substantially 244with the increase of temperature especially above 180°C. As a result, biomass components follow separate reaction mechanisms and produce different products depending on temperature. Hydrothermal processes, in inert atmosphere, can be categorized into three major methods based on temperature or desired product. Pretreatment of biomass at temperature around 140-200°C is often used to facilitate enzymatic hydrolysis for ethanol production. Biomass pretreatment is covered extensively in the earlier sections of the book. Therefore, hydrothermal processes rather than pretreatment are discussed in this chapter. Hydrothermal carbonization (HTC) is the mildest process, where biomass is converted into carbon- rich solid fuel at temperature around 180-280oC under water saturation pressure. The process occurred above HTC temperature and below critical temperature (374oC) is known as hydrothermal liquefaction (HTL), where biocrude is the desired product. Finally, supercritical water gasification (SCWG) of biomass takes place in supercritical region of water to produce methane, synthesis gas, or hydrogen gas, depending on SCWG temperature and applied catalysts. Unlike the HTC, HTL, SCWG, where the reaction is kept under inert atmosphere to prevent oxidation, both wet air oxidation (WAO) and supercritical water oxidation (SCWO) apply excess oxygen pressure to combust feedstocks and produce heat. Both WAO and SCWO are also applied for destruction of toxic, hazardous, and organic pollutants. This chapter covers the fundamentals of HTC, HTL, SCWG, WAO, and SCWO processes based on the current knowledges. Reaction mechanism, chemistry, and kinetics are compared for these hydrothermal processes. The economic and environmental impacts of hydrothermal processes based on the current state-of-the-art knowledge are discussed in the later part of the chapter.
